The invention relates to a process and a device for soldering electrical components at soldering locations, which are provided with soldering material, on a plastic sheet provided with applied conductor tracks.
Plastic sheets with applied conductor tracks and electrical components soldered thereto can advantageously be used if electrical lines are laid in a manner in which the corresponding substrate must have a certain flexibility.
For these reasons, plastic sheets have already been used as substrates for conductor tracks and electrical components which must have a suitable thermal stability on account of the temperatures required during soldering of the electrical components, which are approximately 210xc2x0 C. Plastics of this type are expensive. In this context, hitherto polyamide (PI) has been used as the basic plastic. Because of the costs associated with this plastic, a plastic sheet produced therefrom is in many technologies unsuitable as a substrate for conductor tracks. This applies in particular in the automotive industry, in which conductor bundles have to be positioned at numerous locations within a motor vehicle, the corresponding substrates being exposed to considerable loads when the conductor tracks are being fitted into a motor vehicle during production.
The attempt to use less expensive plastics as substrates for conductor tracks has hitherto failed because technically suitable and sufficiently inexpensive plastics do not withstand the temperatures which occur during soldering. The basic object is to configure the process outlined in the introduction in such a way that it is suitable for known, relatively inexpensive plastics, such as polyester derivatives (PEN, FEPT), without these plastics being impaired in any way during soldering.
According to the invention, this is achieved by the fact that the plastic sheet is heated, from the side which is remote from the components, to a below its damage temperature and after heating its side which is remote from the components is thermally insulated, and then the side which faces the components is acted on by a heating-gas stream which is concentrated onto the locations which are to be soldered by a template which has windows.
The result of this process is that the supply of heat to the plastic sheet is limited to a minimum and only reaches a relatively high level, allowing melting of the solder, where soldering material is already present. This is achieved by the fact that, first of all, as a result of the plastic sheet being heated to a level which is just below its damage temperature, the total energy which is to be supplied to the plastic sheet is considerably limited. Also, on account of the heating of the plastic sheet and on account of its low thermal conductivity and, owing to the temperature difference between plastic sheet and melting point of the solder, only relatively little heat with minimal energy is sufficient. The greater heat which is then to be applied at the soldering locations and is generated by the heating-gas stream can be concentrated to such an extent by the template which has windows that only the locations which are to be soldered are exposed to a high incoming heat flux, with the result that the surrounding regions of the plastic sheet are exposed to scarcely any additional heating, and therefore there can be no damage to the plastic sheet as a result of considerable heating. In order in the process to prevent heat from flowing out of the heated plastic sheet, which would lead to additional, undesirable demand for heat by the plastic sheet, after the plastic sheet has been heated its side which is remote from the components is thermally insulated. As long as this thermal insulation exists, therefore, the plastic sheet retains its temperature which has been produced by heating.
It should also be noted that in International Patent Application WO 94/21415, in connection with reflow soldering of printed circuit board assemblies, the heating of the soldering locations takes place by radiation in a heating chamber. To exclude selected regions in the heating chamber from the radiation, in this document a mask substrate is proposed, which can be displaced out of a position outside the heating chamber into the heating chamber and can be lowered onto the printed circuit board assembly, with the result that the selected region is shadowed from the radiation for a predetermined time. This soldering method is not suitable for plastic sheets, on account of the high introduction of heat which occurs therein, since the polyester derivatives which can be used by the invention would immediately be melted in the heating chamber which is heated by the radiation.
To concentrate the heat supply operation as much as possible and therefore to make it as short as possible, the heating of the plastic sheet and the application of heat to that side of the plastic sheet which faces the components are expediently configured in such a way that this takes place substantially simultaneously.
To carry out the soldering operation, in practice the temperature of the heating-gas stream, with the plastic sheet held securely, is regulated in such a manner that initially the soldering locations are preheated for a time which activates the soldering material, whereupon the temperature of the heating-gas stream is briefly increased until the solder contained in the soldering material melts, and is then rapidly reduced. This procedure ensures that only a minimal time is required for the actual soldering, which takes place during melting of the solder contained in the soldering material, since before the soldering material is melted, the preheating of the soldering locations takes place for a time which activates the soldering material, so that, therefore, only a very short burst of temperature is required in order to melt the solder, with the result that the plastic sheet is reliably protected against excessive heating.
Reducing the temperature of the heating-gas stream after soldering is expediently effected by the heating-gas stream being switched off after the melting. Furthermore, it is also possible to convert the heating-gas stream into a cooling stream, in order to ensure particularly rapidly that no damaging heat whatsoever can remain in the region of the plastic sheet.
A device for carrying out the process described above is expediently configured in such a way that it has a heating blower, in front of which the plastic sheet together with the components held thereon can be positioned, and a heating plate for preheating the plastic sheet a template is arranged in such a manner that it can be pushed in on that side of the plastic sheet which is remote from the heating-gas blower, and the heating plane is arranged displaceably with regard to its distance from the plastic sheet. When the abovementioned thermal insulation of the plastic sheet is then to take place, this expediently occurs by increasing the distance between the heating plate and the plastic sheet, resulting in a larger gas-filled space which acts as insulation. In order for the energy also to be concentrated with regard to the locations to be acted on on the part of the heating blower, the heating-gas blower in expediently provided with positionable outlets, with the result that it is possible to achieve any desired flow concentration at selected locations, and in particular, of course, at the locations which are to be soldered. For the template, it is advantageously possible to use an areal material which has a low thermal conductivity. In this case, the template can easily be pressed onto the plastic sheet and hold the latter in place during the soldering process. However, it is also possible for the template to be formed from a metal sheet which in this case, on account of the high thermal conductivity of metal, is to be held at a distance from this plastic sheet. Furthermore, it is possible for the template to be provided with an interior for accomodating a cooling medium. In this way, it is possible to ensure, by means of the cooled template, that those regions of the plastic sheet which are covered by the template are additionally provided with particular protection on account of the low temperature thereof.